An EBAL system employing an array or fly's eye type electron beam optical column for use in fabricating semiconductor integrated circuits was first described in an article entitled "Integrated Circuit Mask Making by Electron Optical Means" by S. P. Newberry, et al. reported in the NEREM Record-1960 on pages 172 and 173. A more complete report of an EBAL system employing a fly's eye electron beam tube was set forth in the IEEE Transactions on Electron Devices, Volume ED-21, No. 9, September, 1974, in an article entitled "Electron Fly's Eye Lens Artwork Camera" by C. Q. Lemmond, et al., on pages 598 through 603. The IEEE article appearing in the September 1974 electron devices issue describes a system wherein lenslet stitching is accomplished by writing test patterns in electron resist on a calibration target, measuring these patterns visually to determine the absolute scale of the patterns and applying corrections to the fine deflection voltages necessary to obtain stitching of the patterns between lenslets. The system was used to write 1.times.1 inch patterns on 2.times.2 inch glass plates which were to be used as masks for subsequent exposure of semiconductor wafers by light. The absolute accuracy which was achieved resulted in .+-.1.25 micro meters (.mu.m) registration accuracy between patterns written on different plates. A number of problems are associated with this method of achieving stitching and registration: (1) the accuracy is limited by the resolution of the patterns written in the resist and by the light optical instruments used to measure the patterns, (2) no means is provided to correct for inaccuracy which occurs because of the nonorthogonal angle which the electron beam makes at the workpiece when exposure is made on workpieces which are not inserted in exactly the same plane as the original calibration workpiece, and (3) the procedure is tedious and time consuming, a factor which further limits accuracy in actual practice. One of the general purposes of the present invention is to provide a means of lenslet stitching which is capable of accuracy to .+-.50 nanometers (nm) or better, which is rapid, which utilizes electronic signals to obtain the necessary information from which to derive the correction voltages and which is entirely under computer control. It is also a feature of the system that it can be extended to provide multi-level pattern registration in those cases in which subsequent exposures of different patterns are to be made on the same workpiece after processing between exposures. Also the system can make corrections for workpieces which are inserted such that the surface which is to be exposed does not lie exactly in the same plane as the plane in which the lenslet stitching grid was measured. Furthermore, the system allows a workpiece to be inserted and exposed in any station of a multi-station EBAL system.
In the above referenced IEEE article of September 1974, a multiplicity of workpieces could be mounted in a rotatable turntable which then could be employed to rotate one of the target workpieces into operating position relative to the electron beam optical column. Thus, after a particular target workpiece had been exposed to electron beam treatment, it could be rotated out of operating position and a new target workpiece rotated into position under the electron beam optical column. The entire turntable mechanism was evacuated so that withdrawal of a previously exposed target workpiece and insertion of a new target workpiece did not require releasing and re-establishing the vacuum between the target surface and the electron beam optical column while changing target workpieces. However, once a workpiece was in position for electron beam exposure it was not moved to a different position relative to the electron beam optical column for additional exposure. Thus, with this EBAL system there was no means provided for overcoming the effect of a flawed lenslet in the lenslet array. This in turn placed extremely stringent requirements on the fabrication of the lenslet array and its associated array deflector assembly, making the use of such a system commercially impractical, since the existence of even one flawed lenslet in an array of say 18.times.18 lenslets could so substantially effect the yield of integrated circuits produced by the EBAL system as to make the manufacture of integrated circuits with such an EBAL system economically impractical. A second objective of the present invention is to overcome this deficiency in the prior art EBAL systems.